Learning outcomes
By the end of this lesson, students should be able to:
- describe experiments comparing good and poor thermal conductors
- explain conduction in all solids using lattice vibrations
- explain the additional role of free electrons in metals
- distinguish conductor, insulator and specific heat capacity
- apply conduction ideas to handles, pans and insulation
9.1 Conduction as energy transfer
Thermal conduction is transfer of energy through a substance without bulk movement of the substance. It occurs when neighbouring particles interact and pass energy from a hotter region to a colder region. The overall direction is down the temperature gradient.
Conduction can occur in solids, liquids and gases, but it is most important in solids because particles are close together. Liquids and gases are generally poor conductors; convection often dominates when they are free to move.
9.2 Lattice vibration mechanism
In a solid, particles occupy positions in a lattice. At the hot end, particles vibrate with greater amplitude and interact with neighbouring particles. These neighbours gain energy and vibrate more strongly, passing energy progressively through the material.
The particles do not travel from the hot end to the cold end. Energy moves through local interactions. This distinction prevents a common incorrect description in which atoms are said to flow through a solid.
Figure 19. Original KG2UNI diagram.
9.3 Free electrons in metals
Metals contain delocalised electrons that can move through the lattice. At the hot region, electrons gain kinetic energy. They move through the metal and collide with ions and other electrons, transferring energy rapidly. This additional mechanism makes metals much better thermal conductors than most non-metals.
The same free-electron model also helps explain good electrical conductivity, but thermal and electrical conduction are different phenomena. A material can be selected for one property while other properties such as strength, corrosion resistance and cost are also considered.
9.4 Comparing conductors experimentally
One method uses rods of equal length and diameter made from different materials. Small pins are attached with equal amounts of wax at equal distances from one heated end. The best conductor transfers energy fastest, melting the wax and causing its pin to fall first.
A fair comparison requires equal dimensions, equal starting temperatures, identical pin positions and the same heating conditions. If rod diameters differ, the rate can differ because cross-sectional area changes, not only material conductivity.
Figure 20. Original KG2UNI diagram.
9.5 Applications and insulation
Cooking pans often have metal bases because energy must conduct quickly from the heater to food. Handles are made from wood or plastic, which are poor conductors. Oven gloves trap air and use fibres that conduct poorly. Air is a useful insulator only when trapped; if it circulates, convection can transfer energy.
Thicker insulation generally reduces the conduction rate because energy must pass through a greater distance. A material with low thermal conductivity is preferred. The syllabus does not require the conductivity equation, so explanations should remain qualitative unless data are provided.
Worked examples
Metal pan and plastic handle
The metal base conducts thermal energy rapidly into the food. The plastic handle is a poor conductor, so less energy reaches the hand.
Wax-pin experiment
The first pin to fall identifies the rod that transferred energy fastest, provided the rods have equal dimensions and receive equal heating.
Trapped air
Foam contains many small air pockets. Air conducts poorly and cannot circulate over large distances, so both conduction and convection are reduced.
Practical focus
Investigation
Use equal rods arranged radially from a heated central block. Fix drawing pins with equal wax spots at equal distances. Heat the centre and record fall times. Wear eye protection, keep hands away from hot metal and allow equipment to cool before touching.
Examination guidance
- In metals, mention both lattice vibrations and free electrons.
- Do not say heat particles travel along the rod. Energy is transferred through interactions.
- A fair comparison needs rods of equal length and cross-sectional area.
Check your understanding
- What is conduction?
- How is energy transferred through a non-metal solid?
- Why are metals usually good thermal conductors?
- Why are pan handles often plastic?
- What must be controlled when comparing rods?
Answers
- Transfer of thermal energy through a substance without bulk movement.
- Neighbouring lattice particles pass energy through vibrations and interactions.
- Mobile delocalised electrons transfer energy rapidly in addition to lattice vibrations.
- Plastic is a poor conductor, reducing energy transfer to the hand.
- Rod dimensions, starting temperature, pin positions and heating conditions.
